Learn Polish: W maju obchodzimy święto narodowe. - In May, we celebrate a national holiday.

The Genitive case is used when you give an exact date, like the first of May (pierwszego maja). When you only want to say that something happens in a certain month, you use the preposition w followed by the Locative case. For maj, the Locative form is maju.

Does the verb obchodzimy have anything to do with chodzić (to walk)?

Yes! It is made of the prefix ob- (around) and chodzić (to walk). While it can literally mean to walk around something, it is the standard Polish verb used for celebrating or observing holidays, birthdays, and anniversaries.

What case is święto narodowe in here?

It is in the Accusative case because it is the direct object of the verb obchodzimy (we celebrate). Since święto is a neuter noun, its Accusative form looks exactly the same as its basic dictionary form.

Why does the adjective come after the noun in święto narodowe?

Most descriptive adjectives go before the noun. However, classifying adjectives that put a noun into a specific, official category, like narodowe (national), generally go after the noun in Polish.